Organic polymers with near-zero uniaxial thermal expansion coefficients

ABSTRACT

There are provided processes and compositions for obtaining organic polymeric materials having near-zero uniaxial thermal expansion coefficients. According to one embodiment, an organic polymer having a negative uniaxial thermal expansion coefficient is treated by thermal processing under conditions sufficient to cause at least a portion of the polymer chains to become misaligned, thereby decreasing the magnitude of this thermal expansion coefficient to nearly zero. According to a second embodiment, one or more organic polymers having a negative uniaxial thermal expansion coefficient are combined with one or more materials having a positive thermal expansion coefficient to yield a composite material having a uniaxial thermal expansion coefficient which is nearly zero. 
     Useful in the practice of the invention are polyacetylenes having a negative macroscopic thermal expansion coefficient, which are obtained by the solid-state polymerization of crystalline monomers containing two or more conjugated acetylene groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the preparation of organic polymeric materials with near-zero uniaxial thermal expansion coefficients.

2. Description of the Prior Art

Materials with near-zero thermal expansion coefficients are required for use in optical applications and in other areas of technology wherein high dimensional stability over a specified temperature range is necessary.

Conventional materials which have low linear thermal expansion coefficients (α), such as metallic alloys and inorganic glasses, have been employed previously. However, metallic alloys such as Invar, which comprises about 64 weight percent iron and 36 weight percent nickel (average α equal to about 2 × 10⁻ ⁶ ° C⁻ ¹ between zero and 100° C), and Superinvar, which comprises from about 62.5 to 64.0 weight percent iron, 30.5 to 34.0 weight percent nickel and 3.5 to 6.0 weight percent cobalt (α = 0 to 0.5 × 10⁻ ⁶ ° C⁻ ¹ at 20° C), are both expensive and extremely heavy. In addition, these alloys are difficult to machine. Moreover, glasses such as vitrious SiO₂ (α = -0.27 × 10⁻ ⁶ ° C⁻ ¹ at -160° C; + 0.1 × 10⁻ ⁶ ° C⁻ ¹ at -60° C; + 0.35 × 10⁻ ⁶ ° C⁻ ¹ at 0° C and +0.50 × 10⁻ ⁶ ° C⁻ ¹ at +60° C) are difficult to machine, heavy and are also very brittle.

Thus, there is a need for a material having high thermal dimensional stability in at least one direction, which is strong, light, easily machined, and inexpensive. Organic polymers are attractive construction materials because of their low densities, low cost, and ease of processing. However, conventional organic polymers do not have high dimensional stability. They creep, i.e., undergo slow and irreversible deformation, under mechanical stress and typically have high thermal expansion coefficients (e.g., linear thermal expansion coefficients of the order of 10⁻ ⁴ ° C⁻ ¹ for anisotropic linear polymers).

The intrinsic thermal expansion coefficient along the chain direction of a linear polymer is typically small and negative, while the thermal expansion coefficients along orthogonal directions are generally large and positive. However, organic polymers as generally produced are not sufficiently free of regions containing misaligned polymer chains, that is, polymer chains non-parallel with the average chain direction. Thus, the large, positive thermal expansion contribution from such regions overwhelms the much smaller negative chain-direction contribution from highly-aligned chains. Consequently, the observed macroscopic thermal expansion coefficient along the average chain direction, that is, the uniaxial thermal expansion coefficient, is positive. While a higher degree of chain alignment can be obtained for some polymers by drawing processes, morphological changes usually occur at relatively low temperatures which result in irreversible dimensional changes. Consequently, these polymers are not suitable for use as structural elements for applications in which high thermal-dimensional stability is required.

SUMMARY OF THE INVENTION

In accordance with the invention, organic polymers having near-zero uniaxial macroscopic thermal expansion coefficients are obtained which are lightweight, have high strength, are creep-resistant, and easily formed or machined. According to one embodiment of the present invention, a polymer having a negative uniaxial thermal expansion coefficient is thermally processed to obtain a polymer with a near-zero uniaxial thermal expansion coefficient. According to a second embodiment of the present invention, one or more polymers having a negative uniaxial thermal expansion coefficient are combined with one or more materials having a positive thermal expansion coefficient to yield a composite material having a near-zero uniaxial thermal expansion coefficient. These polymers and composites are particularly useful in those areas of application wherein it is desired to substantially eliminate dimensional instability in a structural element.

It has been discovered that polyacetylenes formed by the solid-state polymerization of crystalline monomer containing two or more conjugated acetylene groups have a negative uniaxial thermal expansion coefficient over a wide temperature range (from below -50° C to the thermal degradation or melting temperature, which is usually between 100 and 500° C). These polymers are lightweight, generally having a density less than 1.4 gm/cm³, have strengths as high as 2.5 × 10⁵ psi, and are highly resistant to creep, and accordingly are preferred as starting material in the present invention.

DETAILED DESCRIPTION

According to the present invention, polymers are provided which have a near-zero uniaxial thermal expansion coefficient (α).

The term "thermal expansion coefficient" is herein defined as that quantity α which in the measurement direction satisfies the relation α = ΔL/(L_(o) ΔT)

where ΔL is the dimensional change from the initial dimension L_(o) which occurs reversibly upon changing the temperature of the material by ΔT. To define α at a particular temperature T, ΔT is taken to be vanishingly small.

"Near-zero" thermal expansion coefficients are herein defined to be |α| < 5.0 × 10⁻ ⁶ ° C⁻ ¹ ; that is, where the absolute value of the observed α is less than about 5.0 × 10⁻ ⁶ ° C⁻ ¹.

A uniaxial thermal expansion coefficient is one measured along the average chain direction.

1. Thermal Processing

According to one embodiment of the process of the present invention, polymers having a negative uniaxial thermal expansion coefficient are thermally processed for a time and at a temperature sufficient to induce chain misaligned fractions in the polymer matrix, thereby resulting in a polymer having a near-zero uniaxial thermal expansion coefficient. This fractional chain misalignment is introduced either during a first order phase transformation or during chemical degradation. The polymer during such a thermal process is subjected to at least one cycle comprising (a) heating from a first temperature, herein termed the "initial temperature", which is below the temperature at which the polymer undergoes a first order phase transformation or chemically degrades, to a second or thermal processing temperature which is either above the temperature at which this first order phase transformation occurs or above the temperature at which chemical degradation occurs or both; (b) holding at the second temperature for a period of time; and (c) cooling to a temperature below the first order phase transition temperature or temperature at which substantial degradation occurs.

The first or initial temperature from which the polymer is to be heated may vary widely, being any temperature between room temperature, (i.e., about 20° C) and either the phase transformation or chemical degradation temperature (typically not higher than about 200° C).

As used herein, the second or thermal processing temperature is intended to define the maximum temperature desired to be obtained during the thermal processing step and is a temperature at or above the temperature at which a first order phase transformation or chemical degradation occurs. The chemical degradation temperature, as used herein, is the lowest temperature at which chemical degradation is observable in one hour by a conventional technique such as either thermal gravimetric analysis or infra-red spectroscopy. The first order phase transition referred to above may be either a reversible or irreversible phase transformation and includes crystallographic phase changes and melting. The temperature of this phase transition may be easily determined by conventional methods such as calorimetry, optical microscopy, or X-ray diffraction. While the temperature at which such a first order phase transformation occurs will vary widely, generally such a phase transformation occurs at a temperature between 50° and 400° C and more usually about 100° to 300° C for organic polymers.

The thermal processing temperature ranges from a value approximately equal to the first order transformation or chemical degradation temperature to about 100° C above that temperature. The thermal processing temperature, like the initial temperature, will vary widely depending upon the polymer being treated, ranging generally from about 50° to 400° C, and preferably from about 100° to 300° C.

The rate at which the polymer is heated from its initial temperature to the desired thermal processing temperature is not critical and varies, depending upon the size and crystal perfection of the polymer being treated, the final desired uniaxial thermal expansion coefficient, the temperature range over which the polymer is to be heated and other factors. Generally, however, a rate of heating of about 0.1° to 10³ ° C per minute is conveniently employed.

The amount of time for which the polymer should be maintained at the desired thermal processing temperature will vary widely, depending upon the size and crystal perfection of the polymer being treated, the final desired uniaxial thermal expansion coefficient, the rate of heating of the polymer to the thermal processing temperature and other factors. Generally, however, the polymer is conveniently maintained at the desired thermal processing temperature for a residence time of from about 0.1 second to five hours.

Following thermal processing of the polymer at the desired temperature for the selected period of time, the polymer is cooled by conventional methods, e.g. air cooling, refrigeration, or quenching in a liquid or gas, to a final temperature, which is a temperature below that temperature at which either the crystallographic phase change or chemical degradation occurs. It is not critical that the final temperature achieved on cooling be identical to the initial temperature from which the polymer was heated during the thermal processing step, and such final temperature will generally range from about -100° C to room temperature (i.e., about 20° C). The rate at which the polymer is cooled to the desired final temperature will vary widely, depending upon the size and crystal perfection of the polymer being treated, the final desired uniaxial thermal expansion coefficient, the thermal processing temperature, the difference between the thermal processing temperature and the final temperature and other factors. Generally, however, this time is conveniently between about 1° to 10⁴ ° C per minute.

Upon completion of the foregoing thermal processing cycle, the uniaxial thermal expansion coefficient of the polymer may be determined by conventional methods, as for example by use of a TMS-1 thermal mechanical analyzer (manufactured by Perkin-Elmer). If the desired near-zero uniaxial thermal expansion coefficient has not been achieved by the single thermal processing cycle, polymers having reversible crystallographic phase changes or which chemically degrade at the thermal processing temperature may again be thermally processed in like manner at a time and temperature sufficient to induce further misalignment into the polymer matrix. While there is no upper limit on the number of thermal processing cycles which may be employed for such polymers, generally from about 1 to 10 thermal processing cycles are employed. In causing the polymer to undergo more than one thermal processing cycle, it is not critical that the same initial temperature, thermal processing temperature or final temperature be employed in subsequent cycles, so long as the initial and final temperatures in each such cycle are below the temperature at which the crystallographic phase change or chemical degradation occurs and so long as the thermal processing temperature in each cycle is above this phase change or chemical degradation temperature. Likewise, it is not critical that such subsequent cycles employ the same residence time at the selected thermal processing temperature. Neither is it necessary to employ in subsequent cycles the same rates of heating and cooling used in the first thermal processing cycle.

The polymer may be heated by a wide variety of methods, such as by conduction or radiation heating effected either by any conventional heating device which is provided with the temperature controls to give the precision heating required, or by less commonly used apparatus such as by laser devices.

The heating of the polymer undergoing thermal processing is normally performed under any nonreactive gaseous of liquid environment, such as gaseous nitrogen or helium. In this case the pressure of this environment is not critical, and atmospheric pressure has been found quite satisfactory. However, a reactive liquid or gaseous environment, such as gaseous oxygen or chlorine, can be used to increase the rate at which chain misalignment is introduced by chemical degradation. Then a higher gas pressure will normally correspond to a higher rate of chemical degradation at a particular temperature, so that shorter residence times at the thermal processing temperature are required in order to introduce the required degree of chain misalignment.

For polymers which evidence no first order phase transformations other than melting and do not chemically degrade below the melting point, the thermal processing temperature ranges from approximately the equilibrium melting point of the lowest melting fraction of the polymer to about 50° C above that temperature. In this case the thermal processing temperature will normally be between 100° and 400° C. Here, the main consideration relating to the thermal processing conditions (temperature, heating and cooling rates, and residence time) is the degree of melting. In order to prevent excessive misalignment, it is necessary that not more than about 20% of the polymer be melted and usually not more than about 5%. In all other respects, the same considerations apply as described earlier.

The polymers having negative uniaxial thermal expansion coefficients which have been found especially useful in the practice of the present invention are highly chain-aligned acetylene polymers (polyacetylenes) obtained by solid-state polymerization of cyclic or acyclic acetylene monomers containing two or more conjugated acetylene groups per monomer molecule, and preferably acetylenic monomers having the structure R₁ --(C.tbd.C)--_(n) R₂, where n is an integer of at least 2 and preferably from 2 to 4. The particular substituent groups on the polyacetylene is not critical for obtaining a negative or near-zero thermal expansion coefficient, as long as the precursor monomer is readily polymerizable in the solid state. The R₁ and R₂ terminal substituents may be polar or nonpolar, organic or inorganic. Examples of effective R₁ and R₂ substituents include polar groups, especially polar groups containing oxygen, sulfur, nitrogen, and/or phosphorus atoms, such as hydroxyl groups, alkoxyl, carboxyl, or esterified hydroxyl groups, urethane groups (--OCONHR₃), urea groups (--NHCONR₃ R₄), amide groups and/or imide groups, as well as the corresponding sulfur and phosphorus analogs. R₃ and R₄ include the substituent groups of R₁ and R₂. R₁, R₂, R₃ and R₄ may also be hydrogen and/or functional groups with 1 to 14 carbon atoms, including hydrocarbon groups. Polyacetylenes which are known include those disclosed in German Pat. No. 1,940,691 (issued Feb. 25, 1971 to E. Fisher and G. Wegner), by R. H. Baughman and K. C. Yee in Vol 12, Journal of Polymer Science, Polymer Chemistry Section, pp. 2467-2475 (1974), and by G. Wegner in Vol. 154, Die Makromolekulare Chemie, pp. 35-48 (1972). Especially preferred polymers possessing negative uniaxial linear thermal expansion coefficients are polydiacetylenes obtained by solid-state polymerization of diacetylene monomers, i.e., compounds having two conjugated acetylene groups per monomer and preferably having on each monomer one terminal group (and preferably two terminal groups) other than hydrogen. Examples of solid-state polymerized diacetylenes employed in the practice of the present invention are obtained from monomers selected from the group consisting of those having the formula (I)

    r.sub.1 --c.tbd.c--c.tbd.c--r.sub.2                        (i)

where R₁ and R₂ are independently selected from the group consisting of

    a. CH.sub.3 --(CH.sub.2).sub.m --

    b. HO(CH.sub.2).sub.n --

c. R₃ NHC-O-(CH₂)_(p) -

wherein m is an integer of 0 to 15,n is an integer of 1 to 15, p is an integer of 1 to 15 and R₃ is a member selected from the group consisting of

    i. C.sub.6 H.sub.5 (CH.sub.2).sub.r --

ii. CH₃ (CH₂)_(r) --and

    111. C.sub.10 H.sub.7 (CH.sub.2).sub.4 --,

where r is an integer of 0 to 11.

Examples of the above diacetylene monomers are:

    __________________________________________________________________________      ##STR1##                                (1)                                   2,4-Hexadiyne-1,6-diol-bisphenylurethane                                        ##STR2##                                (2)                                   2,4-Hexadiyne-1,6-diol-monophenylurethane                                       ##STR3##                                (3)                                   2,4-Hexadiyne-1-ol-ethylurethane                                                ##STR4##                                (4)                                   2,4-Hexadiyne-1,6-diol-bisethylurethane                                         ##STR5##                                (5)                                   2,4-Hexadiyne-1,6-diol-bisbutylurethane                                         ##STR6##                                (6)                                   2,4-Hexadiyne-1,6-diol-bishexylurethane                                         ##STR7##                                (7)                                   2,4-Hexadiyne-1,6-diol-bisoctylurethane                                         ##STR8##                                (8)                                   2,4-hexadiyne-1,6-diol-bisdodecylurethane                                       ##STR9##                                (9)                                   2,4-Hexadiyne-1,6-diol-bisnaphthylurethane                                      ##STR10##                               (10)                                  3,5-Octadiyne-1,8-diol-bisphenylurethane                                        ##STR11##                               (11)                                  5,7-Dodecadiyne-1,12-diol-bismethylurethane                                     ##STR12##                               (12)                                  5,7-Dodecadiyne-1,12-diol-bisethylurethane                                      ##STR13##                               (13)                                  5,7-Dodecadiyne-1,12-diol-bisphenylurethane                                     ##STR14##                               (14)                                  3,5-Octadiyne-1,8-diol-bisethylurethane                                         ##STR15##                               (15)                                  3,5-Octadiyne-1,8-diol-bismethylurethane                                        ##STR16##                               (16)                                  3,5-Octadiyne-1,8-diol-bisnaphthylurethane                                      ##STR17##                               (17)                                  4,6-Decadiyne-1,10-diol-bisethylurethane                                        ##STR18##                               (18)                                  4.6-Decadiyne-1,10-diol-bismethylurethane                                       ##STR19##                               (19)                                  4,6-Decadiyne-1,10-diol-bisphenylurethane                                       ##STR20##                               (20)                                  4,6-Decadiyne-1,10-diol-bisnaphthylurethane                                    __________________________________________________________________________

Examples also exist of monomers containing more than two conjugated acetylene groups and cyclic monomers containing one or more diacetylene groups, which can be polymerized in the solid-state to produce highly chain-aligned polymers. These polymers will have either a negative or a near-zero uniaxial thermal expansion coefficient and are therefore suitable for the present application. For example, the monoclinic triyne R--C|C--₃ R where R is --CH₂ OCONHC₆ H₅, is known to undergo 1,4-addition solid-state reaction to produce a three-dimensionally ordered polymer; see, e.g., J. Kiji et al., Vol. 14, Polymer, pp. 433-439 (1973). Similarly, the tetrayne R --C.tbd.C--₄ R, where R is --CH₂ OCONHC₂ H₅, can be thermally polymerized to form a highly chain-aligned 1,4-addition polymer. Likewise, the chain-reinforced polymer derived by the 1,4-addition solidstate reactions of the cyclic monomers (CH₂)₂ -C.tbd.C-C.tbd.C-(CH₂)₂ -₄ and ##STR21## (see, e.g., U.S. Ser. No. 497,027, filed Aug. 13, 1974, now U.S. Pat. No. 3,923,622, issued Dec. 2, 1975, and R. H. Baughman et al., Vol. 12, Journal of Polymer Science, Polymer Chemistry Section, pp. 2467-2475--(1974)) are satisfactory for the present applications. Because of the backbone-reinforced structures of these polymers, the chain conformational fluctuations which produce a negative thermal expansion coefficient will be damped, so as to result in much smaller magnitude coefficients than observed for non-reinforced structures.

The acetylenic monomers useful in preparing the polyacetylenes which may be employed in the practice of the present invention are obtained by conventional methods. Thus, the monomers of formula (I) may be obtained by reacting the corresponding diyne-mono-ol or diyne-diol with the requisite isocyanate having the formula R₃ NCO, where R₃ is as defined above. See e.g., the methods of preparation disclosed by Wegner, Vol. 24 Zeitschrift fur Naturforschung, pp. 824-832 (1969).

The acetylenicmonomers may be polymerized by conventional solid-state polymerization methods, such as by exposing the monomers to heat, mechanical stress or actinic radiation (e.g., light, ultraviolet radiation or γ-ray radiation).

Chain misalignment of the polyacetylenes is introduced most conveniently by the thermal processing techniques disclosed above. For example, for polyacetylenes for which a reversible first-order crystallographic phase transition exists (such as the monoclinic polydiacetylene with substituent groups --(CH₂)₄ OCONHC₂ H₅), chain misalignment can be progressively introduced by cycling through this transition. For a polymer (such as the triclinic dioxane-containing phase of the polydiacetylene with substituent groups --CH₂ OCONHC₆ H₅) for which a non-reversible first-order phase transition exists, the degree of chain misalignment introduced at this transition can be controlled to an extent by varying the rate of the transition. Alternately, a method of general applicability can be used to introduce structural disorder into the solid-state synthesized polyacetylenes. Because the present polymers have fully conjugated and therefore relatively inflexible backbones, they usually thermally degrade at a temperature below their melting points. This degradation can be used to introduce a small amount of chain misalignment into a polyacetylene, and consequently to substantially decrease the magnitude of the thermal expansion coefficient.

2. Composites

In accordance with a second embodiment of the process of the present invention, a polymer having a negative uniaxial thermal expansion coefficient may be combined with a material having a positive thermal expansion coefficient to provide a composite material having a near-zero uniaxial thermal expansion coefficient. In the practice of this embodiment of the present invention, the polymers having negative thermal expansion coefficients which may be employed are coextensive with those polymers discussed previously which may be treated by thermal processing to achieve a near-zero uniaxial thermal expansion coefficient. To be combined with such negative α polymers are materials possessing positive α. Most glasses, ceramics, metals and organic polymers are known to possess a positive α. Of course, one or more polymers having a negative α may be combined with one or more positive α materials, although the present invention may be practiced successfully employing only one material possessing each type of expansion behavior.

The composites may be formed by conventional methods, such as by laminating a polymer of negative α to a material of positive α in an alternating fashion so as to provide a laminate which possesses a near-zero uniaxial thermal expansion coefficient. Similarly, a composite can be formed by incorporating into the positive expanding matrix an acetylenic monomer, and exposing the resulting monomer-containing matrix to heat, mechanical stress or actinic radiation to polymerize the monomer. Thus, there is provided a composite comprising a positive expanding matrix in which is contained the desired quantity of aligned polyacetylene crystals having a negative α in an amount sufficient to provide a composite having a near-zero uniaxial thermal expansion coefficient. Likewise, a positive α material can be incorporated into the matrix of a negative α polymer, such as by incorporating the positive α material into a melt of the desired polyacetylene monomer, and then forming a monomer crystal as by standard melt growing techniques, the resulting crystal containing the desired quantity of positive α material therein. This composite crystal may then be exposed to heat, mechanical stress or actinic radiation to polymerize the monomer, thereby resulting in a polyacetylene crystal containing positive α material incorporated therein in an amount sufficient to provide a near-zero uniaxial thermal expansion coefficient for the composite crystal. Alternately, a positive expanding material, such as an evaporated metal film, may be deposited on the surface of a fiber having a negative expansion coefficient in the fiber direction, so as to obtain a composite fiber having a near-zero expansion coefficient in the fiber direction.

In the foregoing methods of forming a composite, if the overall observed uniaxial thermal coefficient of the composite is negative and not sufficiently low, then the composite may be treated by a thermal process, such as disclosed above, so as to introduce misalignment into the highly aligned negative α polymer fraction, thereby resulting in a decrease of the observed uniaxial thermal expansion coefficient to provide the near-zero α desired for the composite.

Methods for obtaining the required alignment of a polyacetylene phase or phases, or of the precursor monomer phase or phases, in the positive expanding matrix are well-known. See e.g., Whisker Technology (A. P. Levitt, Editor, Wiley-Interscience, NY, 1970) and the references given therein.

Obvious variations of the foregoing techniques may be employed, such as by providing a composite material having positive and negative expansion coefficient materials bonded to each other in series in the structural utilization direction by known bonding techniques.

For the latter case where either the dimension of the bonded region is very small compared with the length of both materials or the bonded region is strain-free, the overall expansion coefficient of the composite is given by the equation α = (α₁ L₁ +α₂ L₂), where L₁ and L₂ are the total lengths of each material in the direction in which the respective thermal expansion coefficients are α₁ and α₂. Consequently, the α of the composite material can be made near-zero at a given temperature t_(o) by choosing the ratio of L₁ to L₂ so that it satisfies the relation ##EQU1## The coefficient α of the composite will remain near-zero for those temperatures in the vicinity of t_(o) for which α₂ (t)/α₁ (t) is nearly equal to α₂ (t_(o))/α₁)T_(o)). If both α₁ and α₂ are given in the temperature region of interest by α_(i) = a_(i) + b_(i) t (where i = 1 or 2), then the region about which α will remain close to zero is increased by choosing the negative α polymer and the positive α material such that either both b₁ and b₂ are small or such that a₁ /b₁ approximately equals a₂ /b₂.

In some instances, the as-polymerized polydiacetylenes include mobile structural defects which must be eliminated by annealing in order to obtain reversible thermal dimensional behavior. Otherwise, the thermal expansion coefficient cannot be properly determined. Accordingly, a low temperature (less than about 100° C) anneal performed for a period of time less than about 1 hr. is satisfactory.

The present information may be further illustrated by reference to the following examples:

Examples

Examples illustrative of preparing a solid-state polymerized acetylenic monomer having a negative uniaxial thermal expansion coefficient are as follows:

1. As described by G. Wegner in Vol 24b, Zeitschrift fur Naturforschung, pp. 824-832 (1969), the monomer 2,4-hexadiyne-1, -6-diol-bisphenylurethane is prepared by the reaction of phenylisocyanate with 2,4-hexadiyne-1,6-diol. This monomer is crystallized in the form of fiber-like crystals approximately 5.0 cm long, using as solvent a 1:1 dioxane-water mixture and employing the growth conditions as reported in the above reference. To avoid fracture of the brittle monomer crystals during their removal from the vessel in which they were grown, they are first irradiated with ultraviolet light (100 watt high-pressure mercury lamp). This irradiation is terminated when the crystals become copper-colored, indicating high conversion to polymer. The increase in strength and degree of flexibility, which result from this treatment, allow removal of the partially polymerized crystals without substantial damage. Polymerization is completed by irradiation with more penetrating radiation as follows. The partially polymerized crystals are irradiated in air at room temperature with ⁶⁰ Co γ-rays at 1 Mrad/hr to obtain total dosages of 15 and 100 Mrad. X-ray diffraction measurements indicate that these polymer crystals are the triclinic modification (Hadicke, et al., Vol. 83, Angewandte Chemie, pp. 253-254 (1971)), which contains 1/2 molecule of interstitial dioxane per monomer unit. The thermal expansion coefficient in the chain direction for these polymer fibers is negative over the entire measurement range from -50° C to 100° C and is given by α = a = bt, where a = -(1.686 ± 0.039) × 10⁻ ⁵ °C⁻ ¹, b = -(1.35 ± 0.18) × 10⁻ ⁷ °C⁻ ², and t is the temperature in °C. The average thermal expansion coefficient in this temperature range is -2.0 × 10⁻ ⁵ °C⁻ ¹, the same result being obtained independently of whether the 15 or the 100 Mrad samples are considered. Also relevant to the utility of these fibers in forming composites of high dimensional stability is the fact that ultimate strengths as high as 2.5 × 10⁵ psi are obtained for individual fibers. The static elastic modulus is high (6.1 ± 0.6) × 10⁶ psi, and no measurable plastic deformation occurs until the high stress levels required for fracture are reached.

2. The monomer 4,6-decadiyne-1, 10-diolbisphenylurethane (prepared by the reaction of 4,6-decadiyne-1,10-diol with phenylisocyanate) is crystallized from an acetone/ water solution as approximately 4 to 5 cm long colorless needles. These crystals are polymerized to high yield by ⁶⁰ Co γ-rays at a dose rate of 1 Mrad/hr to obtain a total dosage of 123 Mrad. After solvent extraction of residual monomer, the polymer needles are metallic in appearance and highly dichroic, with axis of dichroism parallel to the needle axis, indicating that the polymer chain axis coincides with the needle axis. X-ray diffraction measurements indicate that these needles are monoclinic single crystals in which the polymer chain axis coincides with the unique crystallographic axis. The thermal expansion coefficient measured in the fiber direction is negative over the entire temperature interval from -50° C to 100° C, with an average value of -9.3 × 10⁻ ⁶ °C⁻ ¹. This polymer has an ultimate strength which exceeds 1.5 × 10⁵ psi for high perfection fibers and does not measureably plastically deform at stress levels lower than the ultimate strength.

3. The monomer 2,4-hexadiyne-1,6-diol-monophenylurethane is synthesized by the unsymmetrical coupling reaction of propyne-1-ol and 2-propyne-1-ol-phenylurethane and is purified by recrystallization of the crude product from ethyl acetate/petroleum ether. Recrystallization of a 0.4 gm sample from 100 ml of a 10.90 acetone/petroleum ether solvent mixture by slow evaporation at room temperature gives approximately 5 cm long needle-like crystals which turn pink upon exposure to light.

Upon irradiation in air at room temperature with ⁶⁰ Co γ-rays at 1 Mrad/hr for a total dosage of 50 Mrad, these monomer crystals polymerize in 78% yield to provide monocrystalline fibers of poly(2,4-hexadiyne -1,6-diol-monophenylurethane). These highly dichroic crystals are orthorhombic, with the axis of dichroism and the polymer chain axis parallel with the fiber axis. The monomer extracted fibers are heated to 100° C for between 5 and 10 minutes in order to anneal out mobile structural defects in the polymer which otherwise provide irreversible thermal dimensional behavior. The annealed polymer fibers have reversible thermal dimensional behavior in the entire measurement range from -50° to 100° C, with a negative thermal expansion. The average thermal expansion coefficient in the fiber direction is -1.33 × 10⁻ ⁵ °C⁻ ¹ between -50 and + 100° C and -1.19 × 10⁻ ⁵ °C⁻ ¹ between -50 and +40° C. Ultimate strengths exceeding 2.5 × 10⁵ psi are obtained for selected fibers of this polymer.

4. The monomer 2,4-hexadiyne-1-olethylurethane is prepared by an addition reaction between 2,4-hexadiyne-1-ol and ethylisocyanate and purified by recrystallization. Needle-like crystals of the monomer are grown from a 0.004 gm/ml solution of the monomer in n-heptane by cooling the solution in a stoppered test tube from 57° C to room temperature at a rate of 1.2°C/hr. These crystals, which are up to 5 cm long, are polymerized by ⁶⁰ Co γ-ray irradiation (15 Mrad at room temperature), and the residual monomer is extracted with acetone. The resulting polymer fibers are highly dichroic, with the axis of dichroism coincident with the fiber direction. X-ray diffraction measurements indicate that these fibers are not three-dimensionally ordered. Although the polymer chains are parallel, the projection of the polymer structure down the chain-axis direction is not periodic. Consequently, the structure of this polymer is analogous to a conventional nematic phase liquid crystal. Nevertheless these crystals do have a negative thermal expansion coefficient in the chain direction over the entire measurement interval from -50° to -40°C, with an average value of -2.50 × 10⁻ ⁵ °C⁻ ¹.

5. The monomer 4,6-decadiyne-1, 10-diol-bisethylurethane is prepared by the reaction of ethyl isocyanate and 4,6-decadiyne-1,10-diol and purified by recrystallization. The monomer is then dissolved in a 1/3 mixture of acetone and petroleum ether to form a 0.0025 gm/ml solution. Using the same crystallization procedure as described in Example 4, needle-like crystals up to 7.5 cm long are grown from this solution. The polymerization method and monomer extraction are as described in Example 4. The resulting polymer fibers have a metallic appearance similar to that of gold. The monomer extracted fibers are heated to 100°C for between 5 and 10 minutes in order to anneal out structural defects in the polymer which otherwise provide irreversible thermal-dimensional behavior. The annealed polymer fibers have reversible thermal-dimensional behavior in the entire temperature range from -50°C to 100°C, with a negative thermal expansion. The average thermal expansion coefficient in the fiber direction is -1.80 × 10⁻ ⁵ °C⁻ ¹ between -50° and +100°C and -1.88 × 10⁻ ⁵ °C⁻ ¹ between -50° and +40°C.

6. The monomer 2,4-hexadiyne-1,6-diol-bis-ethylurethane is synthesized in 95% yield by the reaction of 11.0 gm (0.10 mole) 2,4-hexadiyne-1,6-diol with 26.5 gm (0.37 mole) ethyl isocyanate in the presence of 1,2-dimethoxyethane (50 ml), dibutyl-tin-2-ethylhexoate (0.1 gm), and triethylamine (2 ml). The monomer is recrystallized as approximately 3 cm long needles by the slow cooling of a 10:90 acetone/petroleum ether solution (75ml) containing 0.2 gm sample, as described in Example 4. These crystals rapidly turn pink upon exposure to light. Polymerization proceeded quantitatively by 60 Co α -ray irradiation at a dose rate of 1 Mrad/hr and a total dosage of 60 Mrad. The resulting polymer fibers are copper-like in appearance and strongly dichroic, with dichroic axis in the fiber axis direction. The polymer fibers are monoclinic monocrystals in which the polymer chain axis, with repeat length 4.9 A, is in a non-unique axis direction. The polymer fibers are annealed as described in Examples 4 and 5, in order to obtain reversible thermal-dimensional behavior. The annealed polymer fiber has a negative thermal expansion coefficient over the entire temperature range from -50° to 100°C. The average thermal expansion coefficient is -1.46 × 10⁻ ⁵ °C⁻ ¹ between -50° and +100°C amd -1.31 × 10⁻ ⁵ °C⁻ ¹ between -50° and +40°C.

7. Crystallization of 1.0 gm of the previously described monomer from a 40:60 water/dioxane solvent mixture (50ml) by solvent evaporation at room temperature gives approximately 3 cm long needles which turn pink upon exposure to light. Upon 50 Mrad irradiation (⁶⁰ Co γ -rays at 1 Mrad/hr), these crystals polymerize quantitatively to produce highly dichroic polymer fibers in which the chain direction and axis of dichroism direction coincide with the needle axis. These fibers have a negative thermal expansion coefficient over the entire temperature range from -50° to 100°C. The average thermal expansion coefficient is -2.08 × 10⁻ ⁵ °C⁻ ¹ between -50° and 100°C and -1.94 × 10⁻ ⁵ °C⁻ ¹ between -50° and +40°C.

Examples illustrative of thermal processing whereby the negative thermal expansion coefficient of a solid-state polymerized acetylenic monomer can be reduced to near-zero value are as follows:

8. A new composition is obtained by annealing the dioxane-containing triclinic phase of the polymer described in Example 1 (poly (2,4-hexadiyne-1,6-diol-bisphenylurethane)) at a temperature between 100° and 300° C for time periods between several seconds and many hours. During annealing, substantially all of the approximately 11.2 weight percent dioxane, which is interstitial in the precursor polymer phase, is volatilized, producing an irreversible phase transition to an apparently less-ordered phase having high ultimate strength and near-zero thermal expansion coefficient along the fiber direction. For example, annnealing the fibers described in Example 1 in nitrogen at a temperature of 165° C for 2 hours results in a 11.7 ± 1% weight loss during dioxane volatilization and an associated first order crystallographic phase transition (evidenced by substantial differences in the X-ray diffraction pattern before and after the anneal).

The average thermal expansion coefficient decreases by nearly an order of magnitude, from -2.0 × 10⁻ ⁵ °C⁻ ¹ between -50° C and 100°C before the anneal to -3.0 × 10⁻ ⁶ °C⁻ ¹ between -50°C and 150°C after the anneal. Surprisingly, the ultimate tensile strength, which coincides with the elastic limit of these annealed fibers, remains extremely high, with measured values as high as 2.5 × 10⁵ psi.

The properties of this material compared with those of a conventional high thermal dimensional stability material such as Invar are superior. For example, the elastic limit, which is crucial from an application viewpoint, is approximately an order of magnitude higher for the annealed fiber than for Invar (20,000 to 30,000 psi for Invar), while the density of the annealed fiber (1.4 gm/cm³) is only about one-sixth of that for Invar (8.0 gm/cm³). This means that in order not to exceed the elastic limit, a structural element utilizing in tension the annealed fiber need only have about 1/10th the volume and 1/60th the weight of a structural element which utilizes Invar.

These important property advantages can be gained without a substantial increase in the low thermal expansion coefficient (about 2.0 × 10⁻ ⁶ °C⁻ ¹ for annealed Invar between 17 and 100°C as compared with (-3.0 ± 1.0) × 10⁻ ⁶ °C⁻ ¹ for the annealed polydiacetylene fiber between -50° and 150°C). Furthermore, the uniaxial thermal expansion coefficient of the annealed fibers is negative and, consequently, can be further reduced by increasing the thermal processing temperature for the precursor polymer, which introduces a larger amount of chain disorder into the polymer by causing the phase transition to occur more rapidly. This method of reducing the thermal expansion coefficient of a solid-sate polymerized acetylenic monomer by the introduction of structural disorder during a phase transition can be readily applied to other polyacetylenes.

9. The composition 2,4-hexadiyne-1,6-diol-bis-phenylurethane crystallizes from methanol solution as long fiber-like crystals which are polymerizable in the solid-state (preferably by exposure to actinic radiation). The corresponding polymer fibers are believed to contain interstitial methanol, which can be volatilized analogously with the process for the dioxane-containing phase so as to produce polymer fibers having high thermal dimensional stability. The phase transition need not be a solid-state transformation, since heating a solid-state synthesized polyacetylene slightly above its melting point for a brief period of time (minutes or less) will cause partial melting. Upon cooling to room temperature, the melted region of the polymer will solidify as poorly chain-aligned or non-aligned regions of the material. These regions which have undergone melting and resolidification, will have a positive theremal expansion contribution which will compensate the negative thermal expansion contribution from chain-aligned regions of the polymer so that the thermal expansion coefficient in the average chain direction of the polymer is near-zero.

10. Thermal degradation can be alternatively used to introduce non-chain aligned regions into the polymer and consequently to reduce the thermal expansion coefficient in the average chain direction to near-zero value. This is demonstrated using fibers of poly(4,6-decadiyne-1, 10-diol-bisphenylurethane), which are discussed in Example 2. The average expansion coefficient for the solid state polymerized fibers which have been monomer-extracted is -9.3 × 10⁻ ⁶ °C⁻ ¹ in the temperature range -50° to 100°C. Thermal annealing these fibers at 240°C for approximately 5 minutes decreases the magnitude of the average thermal expansion coefficient to -3.1 × 10⁻ ⁶ °C⁻ ¹. Annealing the initial fibers for a longer period of time (30 min at 235°C) results in a fiber which has a positive average thermal expansion coefficient in this temperature range with value + 3.1 × 10⁻ ⁶ °C⁻ ¹. For many applications, these thermal expansion coefficients are close enough to zero to be negligible. Since either small negative or small positive thermal expansion coefficients can be obtained using the above annealing processes, this demonstrates that by changing the anneal time, the thermal expansion coefficients can be further reduced to an arbitrarily small magnitude. For instance, by annealing at 240°C for a slightly longer period of time or by annealing at 235°C for a slightly shorter period of time, the magnitude of the thermal expansion coefficient would be further reduced.

Examples illustrative of polyacetylenes useful for combining with positive α materials to form composites having a near-zero thermal expansion coefficient are as follows:

11. For the specific polyvinylidene fluoride polymer investigated by C. E. Weir, Vol. 53, Journal of Research - National Bureau of Standards, pp. 245-252 (1954), α₁ = a₁ + b₁ t = 1.01 × 10⁻ ⁴ °C⁻ ¹ + 8.0 × 10⁻ ⁷ °C⁻ ² t in the temperature range of from 20° to 90° C. On the other hand, the thermal expansion coefficient of the dioxane-containing phase of poly (2,4-hexadiyne-1,6-diol-bisphenylurethane), is given by α₂ = a₂ + b₂ t = -1.69 × 10⁻ ⁵ C -1.35 × 10⁻ ⁷ °C⁻ ² t. Since in this case a₁ /b₁ = 1.26 × 10⁻ ² °C, which is nearly equal to a₂ /b₂ = 1.25 × 10⁻ ² °C, it is possible to match the negative thermal expansion coefficient of the polydiacetylene mentioned above with the thermal expansion coefficient of the positive expanding material over a wide temperature range in which the individual coefficients are well-represented by the a_(i) and b_(i) given above. Thus, a composite material may be obtained having a near-zero uniaxial thermal expansion coefficient over this temperature range.

In like manner, the polymers produced in Examples 1 through 7 are combined with positive α materials to obtain composite materials having a near-zero unaxial thermal expansion coefficient. 

What is claimed is:
 1. A process for obtaining an organic polymer having a near-zero uniaxial thermal expansion coefficient, characterized in that a highly chain alinged polyacetylene obtained by solid-state polymerization of an acetyleneic monomer selected from the group of cyclic and acyclic acetylenic monomers containing at least two conjugated acetylene groups per monomer molecule and having a negative uniaxial thermal expansion coefficient is subjected to at least one cycle comprising (a) heating from a first temperature to a second temperature, the first temperature being below and the second temperature being above the lowest temperature at which a first order phase transformation or chemical degradation occurs, (b) holding at the second temperature for a period of time sufficient to induce sufficient chain misaligned fractions in the polyacetylene to produce a uniaxial thermal expansion coefficient therein having an absolute value of less than about 5.0 × 10⁻ ⁶ °C⁻ ¹ and (c) cooling to a temperature below the firt-order phase transition temperature or temperature at which substantial chemical degradation occurs.
 2. The process of claim 1 in which the first temperature ranges from about 20° to 200° C.
 3. The process of claim 1 in which the second temperature ranges from a value approximately equal to the first order transformation or chemical degradation temperature to about 100° C above that temperature.
 4. The process of claim 1 in which the polymer is heated from the first temperature to the second temperature at a rate of about 0.1° to 10³ ° C per minute.
 5. The process of claim 1 in which the polymer is held at the second temperature for a period of time of about 0.1 second to 5 hours.
 6. The process of claim 1 in which the polymer is cooled to temperature ranging from about -100° to 20° C.
 7. The process of claim 1 in which the polymer is cooled at a rate of about 1° to 10⁴ °C per minute.
 8. The process of claim 1 in which the second temperature ranges from a value approximately equal to the temperature corresponding to the equilibrium melting point of the lowest melting point fraction of the polymer to about 50° C above that temperature.
 9. The process of claim 1 in which the polymer is heated and cooled from 1 to 10 cycles.
 10. The process of claim 9 in which the polymer is heated and cooled for 1 cycle.
 11. The process of claim 1 in which the acetylene monomers have the structure R₁ --C.tbd.C--_(n) R₂, where n is an integer of at least 2 and R₁ and R₂ are selected from the group consisting of hydroxyls, alkoxyls, carboxyls, esterified hydroxyls, urethanes, ureas, amides and imides.
 12. The process of claim 11 in which n is an integer from 2 to
 4. 13. The process of claim 12 in which the polymer is a polydiacetylene obtained by solid-state polymerization of a diacetylene monomer.
 14. The process of claim 11 in which R₁ and R₂ are independently selected from the group consisting of

    a. CH.sub.3 --(CH.sub.2).sub.m --

    b. HO(CH.sub.2).sub.n --

    c. R.sub.3 NHOCO(CH.sub.2).sub.p --

where m is an integer of 0 to 15, n is an integer of 1 to 15, p is an integer of 1 to 15 and R₃ is a member selected from the group consisting of

    i. C.sub.6 H.sub.5 (CH.sub.2).sub.r --

    ii. CH.sub.3 (CH.sub.2).sub.r -- and

    iii. C.sub.10 H.sub.7 (Ch.sub.2).sub.r --

where r is an integer of 0 to
 11. 